There are various forms of treatment that act by means of radiation on a material to be machined. For instance, ultraviolet radiation can trigger a local crosslinking reaction after injection of a photosensitiser. For precise incisions, an accurate localisation of the beam focus of a laser radiation provided by the radiation-source at the desired position of the incision is also required. For this purpose the use of interface units has proved useful, by means of which the object to be irradiated is capable of being positioned with respect to a laser cutting device including the radiation-source.
The materials that are capable of being machined with the laser cutting device may, in principle, be of any nature. They may be dead matter or living (biological) material. An exemplary and by no means limiting field of application of the interface units under consideration here lies in laser-surgical ophthalmology, in which incisions (e.g. individual incisions or complex incision figures) are to be generated in the cornea or in other tissue parts of the human eye by means of focused laser radiation. This field includes, for example, fs LASIK (femtosecond laser in-situ keratomileusis), in which a small disc generally designated in specialist circles by the English term ‘flap’ is cut out of the anterior region of the cornea by means of ultra-short-pulse laser radiation. The pulse durations that are used customarily are of the order of magnitude of femtoseconds—hence the name fs LASIK. The pulses are focused beneath the anterior surface of the cornea in the interior of the tissue. By positioning the focal points in a desired incision surface, as a result the flap is cut out of the cornea. The flap remains connected to the cornea at a peripheral point and is folded aside for a subsequent ablation (resection of tissue by means of laser radiation) of underlying corneal tissue. After implementation of the ablation, the flap is folded back, and a relatively rapid healing takes place, with the corneal surface remaining largely intact.
It will be understood that the interface units under consideration here may also find application in any other treatment techniques that require generation of an incision in corneal or other ocular tissue. It will be understood, furthermore, that the interface units under consideration within the scope of the invention may also come into operation in other applications that serve for the machining of other forms of biological tissue and even for the machining of dead matter with radiation, above all laser radiation.
In the case of medical applications, particularly in ophthalmology, for reasons of process engineering and hygiene it is additionally necessary that the interface units are sterile articles which are employed anew for each intervention or even for each incision. At the same time, especially in the case of eye operations, particularly stringent demands have to be made of the cutting precision. For a high degree of cutting precision, in addition to an application-dependent form of the beam focus (in shape and size) a high degree of positioning accuracy of the beam focus in the target material is crucial. In the case of eye treatments, for example, a cutting precision of at most a few micrometres, preferably less than 5 μm, is striven for. Ideally, the tissue incision should be able to be placed with an inaccuracy of no more than 1 μm or 2 μm.
For the precision in the depth of incision that is being striven for, a high manufacturing accuracy of the interface units is required. Particularly in ophthalmology, the disposable character and the resulting necessity to provide the interface units in large numbers represent a great challenge for the achievement of the low manufacturing tolerances that are needed. An attempt may be made to predetermine the geometrical dimensions of the interface units. Since the geometrical manufacturing tolerances of the interface units then enter directly into the inaccuracy of the depth of the incision in the tissue, an accurate reproducibility from piece to piece with respect to these geometrical dimensions has to be guaranteed in the course of production of the interface units. For this purpose, on the basis of a reference interface unit a requisite distance between a reference surface facing towards the laser cutting device and a reference surface facing towards the tissue to be treated may, for example, be predetermined and be put into practice uniformly in the production process within the bounds of the manufacturing tolerance.